Investment casting processing is particularly useful for casting where close tolerances or intricacy of design are factors. One example has been in the casting of airfoils such as turbine blades and vanes made from specialty alloys and subject to high temperature service. Investment casting permits casting of thin sections, such as the airfoil portion of a turbine blade.
Solidification of castings, including investment castings typically occurs through the mold walls, as heat is withdrawn from the casting. This solidification normally occurs through the casting walls, which transfer heat from the molten metal in the casting to the ambient atmosphere. As heat is withdrawn, nucleation sites form on the mold walls and solidification fronts grow into the molten metal as dendrites.
Grains also are heterogeneously nucleated by solid fragments in front of the solid/liquid interface. The number of these solid fragments is proportional to the amount of undercooling. The morphology of the nucleated grains is determined by the direction and the amount of heat flux at any given time.
What is needed is a casting system that permits additional controls over the solidification of the metal or metal alloy during solidification to homogenize temperature distribution, reduce segregation and break/distribute volumetric imperfections in the casting, when required.